1. Field of the Invention
The present invention relates to cold cathode fluorescent lamps used for, for example, illuminating light sources and backlights of liquid crystal displays for monitors of personal computers, liquid crystal televisions, and car navigations, etc. In particular, the present invention relates to a production method for an electrode which is advantageously applied to the above cold cathode fluorescent lamps.
2. Description of the Related Art
As shown in FIG. 1, a cold cathode fluorescent lamp has a structure in which electrodes 3 are disposed at both ends of a glass tube 1 and are electrically connected to outer devices via terminals 2. A phosphor 4 is coated on an inner surface of the glass tube 1, and a filler gas 5 composed of a rare gas and a small amount of mercury (Hg) is filled in the glass tube 1. In the cold cathode fluorescent lamp, high electric field is applied to the electrodes 3 disposed at both ends of the glass tube 1, and a glow discharge is generated in a low-pressure mercury vapor. As a result, ultraviolet rays are generated by mercury excited by the glow discharge, and the phosphor 4 filled in the glass tube 1 is excited by the ultraviolet rays, thereby emitting light. Cup-shaped electrode which can provide hollow cathode effect have been used as the electrode 3 in recent years. In this case, the terminals 2 are bonded to bottoms of the cup-shaped electrodes 3 by brazing or the like, and the terminals 2 and the electrodes 3 are integrally formed.
The cold cathode fluorescent lamp having the above structure has been applied to backlights of liquid crystal displays in recent years. Recently, the above cold cathode fluorescent lamp has been applied to liquid crystal televisions and liquid crystal displays for car navigations, and has been widely used. The number of the cold cathode fluorescent lamp of the display having 15 inches or less is usually one for a product. However, required luminance cannot be obtained when the cold cathode fluorescent lamp is applied to large monitors, televisions, or the like, so that plural cold cathode fluorescent lamps are used. Therefore, demand to the cold cathode fluorescent lamp has increased rapidly.
While demand to cold cathode fluorescent lamps is increasing, the following items (1) to (5) are required in cold cathode fluorescent lamps and electrodes used therefore to improve performance of liquid crystal displays.
(1) From requirements of making products to be thin and light, cold cathode fluorescent lamps are required to have a small diameter, and electrodes are required to be further small and superior in forming in accordance with the requirement in the cold cathode fluorescent lamps.
(2) In liquid crystal displays, the improvement in a contrast ratio thereof is required, and high luminance of cold cathode fluorescent lamps is required. Luminance of the cold cathode fluorescent lamps substantially increases in proportion to inner diameters thereof. However, since the cold cathode fluorescent lamps have been improved to be small, the electrodes are required to be composed of a material having good electric discharge characteristics, that is, a material having low cathode drop voltage.
(3) In accordance with requirements in lower power consumption of products, lower power consumption of cold cathode fluorescent lamps is required. In the requirement in lower power consumption of cold cathode fluorescent lamps, the electrodes are required to be composed of a material having lower cathode drop voltage so as to emit light in an intensity level higher than that of the conventional technique.
(4) Since service life of a cold cathode fluorescent lamp is essential factor of service life of a product, service life of a cold cathode fluorescent lamp is required to be longer. Therefore, electrodes are required to be composed of a material which is difficult to be sputtered when amount of electric discharge increases.
(5) Since the market competition in price among providers of liquid crystal displays is intensified, high price products cannot be sold even if the products satisfy the above items (1) to (4). Therefore, the production cost of the products is required to be low.
In conventional techniques, since nickel has low cathode drop voltage and is easily machined, Ni has been used as an electrode material for cold cathode fluorescent lamps. However, in nickel electrodes, if amount of electric current is increased to increase amount of electric discharge and thereby obtain a high luminance of a cold cathode fluorescent lamp, the temperature of the lamp rises, and the mercury pressure is too high, so that light flux is saturated. The increase in applying voltage causes increase in consumption power. Therefore, materials having lower cathode drop voltage are required to apply to electrodes instead of nickel.
As disclosed in Japanese Unexamined Patent Application Publication No. H10-144255 and Japanese Unexamined Patent Application Publication No. 2002-289138, a material layer having a work function lower than that of nickel is provided on an inner surface of a cup-shaped electrode for cold cathode fluorescent lamps, so that amount of electric discharge is increased. However, the manufacturing process of the electrode requires a treatment to coat the material layer having a low work function. Since the substrate of the electrode is composed of nickel, wear degree of the electrode cannot be improved, and all of the above items (1) to (5) cannot be satisfied.
In order to solve the above problems, a metal having a low work function and a high melting point and being difficult to sputter has been researched, and molybdenum is started to apply to electrode materials. Tungsten having much higher melting point than molybdenum has been researched to apply to electrode materials.
An electrode composed of molybdenum for cold cathode fluorescent lamps is formed by punching and deep drawing a rolled plate into a cup-shape. Since molybdenum has a melting point higher than nickel and electric discharge characteristics better than nickel, the molybdenum electrode satisfies the above items (1) to (4). A molybdenum electrode having an outer diameter of about 1.5 to 3.0 mm and a thickness of about 0.1 to 0.3 mm has been produced. However, rolled plates of molybdenum readily have anisotropic and insufficient ductility, thereby being difficult to be subjected to plastic working. In addition, yield efficiency of the molybdenum plate is not good, whereby production cost of the electrode is high, and the above item (5) is not satisfied. Furthermore, the thickness ratio of a cylindrical portion and a bottom portion of the molybdenum electrode is usually about 1:2 by punching and deep drawing, whereby option of design for electrodes is restricted.
Regarding application of tungsten to an electrode for cold cathode fluorescent lamps, since tungsten is hard and insufficient in ductility, tungsten cannot be subjected to deep drawing, whereby the tungsten electrode has not been on mass production.